diff --git a/Sources/Testing/ABI/ABI.swift b/Sources/Testing/ABI/ABI.swift
index 1707953a0..7a33970fc 100644
--- a/Sources/Testing/ABI/ABI.swift
+++ b/Sources/Testing/ABI/ABI.swift
@@ -45,7 +45,8 @@ extension ABI {
   /// The current supported ABI version (ignoring any experimental versions.)
   typealias CurrentVersion = v0
 
-  /// The highest supported ABI version (including any experimental versions.)
+  /// The highest defined and supported ABI version (including any experimental
+  /// versions.)
   typealias HighestVersion = v6_3
 
 #if !hasFeature(Embedded)
@@ -93,6 +94,39 @@ extension ABI {
 #endif
 }
 
+/// The value of the environment variable flag which enables experimental event
+/// stream fields, if any.
+private let _shouldIncludeExperimentalFlags = Environment.flag(named: "SWT_EXPERIMENTAL_EVENT_STREAM_FIELDS_ENABLED")
+
+extension ABI.Version {
+  /// Whether or not experimental fields should be included when using this
+  /// ABI version.
+  ///
+  /// The value of this property is `true` if any of the following conditions
+  /// are satisfied:
+  ///
+  /// - The version number is less than 6.3. This is to preserve compatibility
+  ///   with existing clients before the inclusion of experimental fields became
+  ///   opt-in starting in 6.3.
+  /// - The version number is greater than or equal to 6.3 and the environment
+  ///   variable flag `SWT_EXPERIMENTAL_EVENT_STREAM_FIELDS_ENABLED` is set to a
+  ///   true value.
+  /// - The version number is greater than or equal to that of ``ABI/ExperimentalVersion``.
+  ///
+  /// Otherwise, the value of this property is `false`.
+  static var includesExperimentalFields: Bool {
+    switch versionNumber {
+    case ABI.ExperimentalVersion.versionNumber...:
+      true
+    case ABI.v6_3.versionNumber...:
+      _shouldIncludeExperimentalFlags == true
+    default:
+      // Maintain behavior for pre-6.3 versions.
+      true
+    }
+  }
+}
+
 // MARK: - Concrete ABI versions
 
 extension ABI {
@@ -125,6 +159,14 @@ extension ABI {
       VersionNumber(6, 3)
     }
   }
+
+  /// A namespace and type representing the ABI version whose symbols are
+  /// considered experimental.
+  enum ExperimentalVersion: Sendable, Version {
+    static var versionNumber: VersionNumber {
+      VersionNumber(99, 0)
+    }
+  }
 }
 
 /// A namespace for ABI version 0 symbols.
diff --git a/Sources/Testing/ABI/Encoded/ABI.EncodedEvent.swift b/Sources/Testing/ABI/Encoded/ABI.EncodedEvent.swift
index 73e7db2ac..c9683f927 100644
--- a/Sources/Testing/ABI/Encoded/ABI.EncodedEvent.swift
+++ b/Sources/Testing/ABI/Encoded/ABI.EncodedEvent.swift
@@ -98,8 +98,12 @@ extension ABI {
       instant = EncodedInstant(encoding: event.instant)
       self.messages = messages.map(EncodedMessage.init)
       testID = event.testID.map(EncodedTest.ID.init)
-      if eventContext.test?.isParameterized == true {
-        _testCase = eventContext.testCase.map(EncodedTestCase.init)
+
+      // Experimental fields
+      if V.includesExperimentalFields {
+        if eventContext.test?.isParameterized == true {
+          _testCase = eventContext.testCase.map(EncodedTestCase.init)
+        }
       }
     }
   }
diff --git a/Sources/Testing/ABI/Encoded/ABI.EncodedIssue.swift b/Sources/Testing/ABI/Encoded/ABI.EncodedIssue.swift
index c1e3c12fd..c593a68a5 100644
--- a/Sources/Testing/ABI/Encoded/ABI.EncodedIssue.swift
+++ b/Sources/Testing/ABI/Encoded/ABI.EncodedIssue.swift
@@ -72,12 +72,14 @@ extension ABI {
         isFailure = issue.isFailure
       }
 
-      // Experimental
-      if let backtrace = issue.sourceContext.backtrace {
-        _backtrace = EncodedBacktrace(encoding: backtrace, in: eventContext)
-      }
-      if let error = issue.error {
-        _error = EncodedError(encoding: error, in: eventContext)
+      // Experimental fields
+      if V.includesExperimentalFields {
+        if let backtrace = issue.sourceContext.backtrace {
+          _backtrace = EncodedBacktrace(encoding: backtrace, in: eventContext)
+        }
+        if let error = issue.error {
+          _error = EncodedError(encoding: error, in: eventContext)
+        }
       }
     }
   }
diff --git a/Sources/Testing/ABI/Encoded/ABI.EncodedTest.swift b/Sources/Testing/ABI/Encoded/ABI.EncodedTest.swift
index 11c309e83..43a1b615b 100644
--- a/Sources/Testing/ABI/Encoded/ABI.EncodedTest.swift
+++ b/Sources/Testing/ABI/Encoded/ABI.EncodedTest.swift
@@ -76,10 +76,6 @@ extension ABI {
     /// The tags associated with the test.
     ///
     /// - Warning: Tags are not yet part of the JSON schema.
-    ///
-    /// @Metadata {
-    ///   @Available("Swift Testing ABI", introduced: 6.3)
-    /// }
     var _tags: [String]?
 
     init(encoding test: borrowing Test) {
@@ -87,18 +83,19 @@ extension ABI {
         kind = .suite
       } else {
         kind = .function
-        let testIsParameterized = test.isParameterized
-        isParameterized = testIsParameterized
-        if testIsParameterized {
-          _testCases = test.uncheckedTestCases?.map(EncodedTestCase.init(encoding:))
-        }
+        isParameterized = test.isParameterized
       }
       name = test.name
       displayName = test.displayName
       sourceLocation = test.sourceLocation
       id = ID(encoding: test.id)
 
-      if V.versionNumber >= ABI.v6_3.versionNumber {
+      // Experimental fields
+      if V.includesExperimentalFields {
+        if isParameterized == true {
+          _testCases = test.uncheckedTestCases?.map(EncodedTestCase.init(encoding:))
+        }
+
         let tags = test.tags
         if !tags.isEmpty {
           _tags = tags.map(String.init(describing:))
diff --git a/Sources/Testing/ABI/EntryPoints/EntryPoint.swift b/Sources/Testing/ABI/EntryPoints/EntryPoint.swift
index a97d33c9e..727d91632 100644
--- a/Sources/Testing/ABI/EntryPoints/EntryPoint.swift
+++ b/Sources/Testing/ABI/EntryPoints/EntryPoint.swift
@@ -57,10 +57,10 @@ func entryPoint(passing args: __CommandLineArguments_v0?, eventHandler: Event.Ha
       // Check for experimental console output flag
       if Environment.flag(named: "SWT_ENABLE_EXPERIMENTAL_CONSOLE_OUTPUT") == true {
         // Use experimental AdvancedConsoleOutputRecorder
-        var advancedOptions = Event.AdvancedConsoleOutputRecorder<ABI.HighestVersion>.Options()
+        var advancedOptions = Event.AdvancedConsoleOutputRecorder<ABI.ExperimentalVersion>.Options()
         advancedOptions.base = .for(.stderr)
 
-        let eventRecorder = Event.AdvancedConsoleOutputRecorder<ABI.HighestVersion>(options: advancedOptions) { string in
+        let eventRecorder = Event.AdvancedConsoleOutputRecorder<ABI.ExperimentalVersion>(options: advancedOptions) { string in
           try? FileHandle.stderr.write(string)
         }
 
diff --git a/Sources/Testing/ExitTests/ExitTest.swift b/Sources/Testing/ExitTests/ExitTest.swift
index 904d3a40a..f1421ee2b 100644
--- a/Sources/Testing/ExitTests/ExitTest.swift
+++ b/Sources/Testing/ExitTests/ExitTest.swift
@@ -541,10 +541,9 @@ extension ABI {
   /// The ABI version to use for encoding and decoding events sent over the back
   /// channel.
   ///
-  /// The back channel always uses the latest ABI version (even if experimental)
-  /// since both the producer and consumer use this exact version of the testing
-  /// library.
-  fileprivate typealias BackChannelVersion = v6_3
+  /// The back channel always uses the experimental ABI version since both the
+  /// producer and consumer use this exact version of the testing library.
+  fileprivate typealias BackChannelVersion = ExperimentalVersion
 }
 
 @_spi(ForToolsIntegrationOnly)
diff --git a/Tests/TestingTests/SwiftPMTests.swift b/Tests/TestingTests/SwiftPMTests.swift
index 672e34a03..e61c3b237 100644
--- a/Tests/TestingTests/SwiftPMTests.swift
+++ b/Tests/TestingTests/SwiftPMTests.swift
@@ -376,7 +376,7 @@ struct SwiftPMTests {
     }
     #expect(testRecords.count == 1)
     for testRecord in testRecords {
-      if version.versionNumber >= ABI.v6_3.versionNumber {
+      if version.includesExperimentalFields {
         #expect(testRecord._tags != nil)
       } else {
         #expect(testRecord._tags == nil)